Tabs lose their URLs
We're on Safari 6.0.2.
Sometimes I keep tabs open in Safari for awhile, before I get around to reading the page. Lately, I've been noticing some tabs lose their URLs. When it happens, it usually happens to several tabs all at once, or all open tabs. Otherwise the browser continues to work normally.
So, the web page's title remains on the tab, and the title displays at the top of the browser window. But the URL bar is empty. "Reload all tabs" does not restore the URL, nor does "Reopen All Windows From Last Session."
The only way I can figure out to re-open the page is to search for the title in a search engine.
Is this a bug or a feature? Can anyone explain why it is happening or what I can do to prevent it?
I've noticed a few similar recent questions on the forum, all un-answered.
Thanks in advance, Tim
MacBook, Mac OS X (10.7.5)